Protection circuit and method for power transistors, voltage regulator using the same

Abstract: The purpose of the present invention is to supply a method and a circuit simple and accurate enough to protect at least one transistor against exceeding a complex limit implying processing of multiple electrical quantities associated with said transistor. Since in many practical cases said complex limit corresponds to the product of at least two quantities, typically a current and a voltage, the circuit in accordance with the present invention generates electrical signals basically proportional to said quantities, multiplies them, compares the product with a reference signal corresponding to the limit placed on the transistor and acts on the transistor in such a way that said limit is not exceeded. Advantageously the multiplication of currents can be provided simply by means of connection in series of bipolar transistor junctions at which said currents are supplied to the respective emitters. In this case it is additionally advantageous to generate the reference signal by means of connection in series of the bipolar transistor junctions in such a manner as to have an analogous behaviour of the multiplier and the generator.

Description: The present invention relates to a method and a protection circuit for power transistors and to a voltage regulator using them. In the electronics field there often arises a need for controlling in certain nodes and/or branches of a circuit, e.g. integrated, the voltages and currents so that these quantities do not exceed maximum values established for design requirements, operating specifications, technological limitations (SOA, primary and secondary breakdown), and bonding. Equally important is control of the power dissipated along certain branches of the circuit which, whether for design specification or limitation of maximum power that can be dispelled by the package containing it, must be limited to a maximum value. Fairly simple and accurate circuits for protecting transistors against exceeding a simple current or voltage limit are well known. UK Patent Application No. GB 2 030 808 discloses a protection circuit for a first transistor operative to control the applied signal by a second transistor if the current through and/or voltage across the first transistor moves out of a safe region. Said protection circuit comprises a first and a second resistors connected to sense the collector-emitter voltage of the first transistor and a third transistor to sense its emitter current. The purpose of the present invention is to supply a method and a circuit sufficiently simple and accurate to protect at least one transistor against exceeding a more complex limit implying processing of multiple electrical quantities associated with the transistor. In the following description reference is made to the protection of a transistor, e.g. the MOS or BJT type, against exceeding the maximum power that can be dispelled by the package containing them but, as will be mentioned, the teaching of the present invention applies also to other types of protection such as e.g. against the secondary breakdown for bipolar junction transistors. Said purpose is achieved by means of the method having the functions set forth in claim 1 and by means of the circuit having the characteristics set forth in claim 5 while further advantageous aspects of the present invention are set forth in the dependent claims. In accordance with another aspect the present invention also concerns a voltage regulator, preferably integrated, in which said circuit finds advantageous application. Since in many practical cases said complex limit corresponds to the product of at least two quantities, typically a current and a voltage, the circuit in accordance with the present invention generates electrical signals essentially proportional to said quantities, multiplies them, compares the product with a reference signal corresponding to the limit set for the transistor and acts on the transistor in such a manner that said limit is not exceeded. Advantageously the multiplication of currents can be provided simply by means of connection in series of bipolar transistor junctions at which said currents are supplied to the respective collectors or emitters. In this case it is further advantageous to generate the reference signal by means of connection in series of bipolar transistor junctions in such a manner as to have analogous behaviour of the multiplier and generator. The invention is clarified by the following description considered together with the annexed drawings wherein - FIG. 1 shows a block diagram of a voltage regulator in accordance with the present invention, FIG. 2 shows a partial electrical diagram of a first embodiment of the circuit in accordance with the present invention, FIG. 3 shows in a voltage-current graph the behaviour of the circuit of FIG. 2 compared with the expected ideal behaviour, FIG. 4 shows the partial electrical diagram of a second embodiment of the circuit in accordance with the present invention, FIG. 5 shows the electrical diagram of current detection means which can be employed in combination with the circuit of FIG. 4, and FIG. 6 shows the electrical diagram of voltage detection means which can be employed in combination with the circuit of FIG. 4. Hereinafter when power transistor is discussed it is intended only to indicate the transistor to be protected and which in general is the power type without being absolutely limited to actual 'power' transistors such as DMOS devices. With reference to FIG. 2 or 4 the method in accordance with the present invention for protecting at least one power transistor PT, in the example of MOS type, having at least one control terminal G (the MOS gate) and two main conduction terminals D and S (respectively the drain and source of the MOS) which identify a main conduction path D-S comprises the phases: a)generation of a first electrical signal S1 essentially proportional to the current flowing in the path D-S, b)generation of a second electrical signal S2 essentially proportional to the voltage across the path D-S (in the example the voltage VDS of the MOS), c)multiplication of at least the signals S1 and S2 to obtain an electrical product signal PS, d)comparison of the signal PS with an electrical reference signal RS finding a difference electrical signal, and e)driving of the control terminal G of the transistor PT by means of the signal DS in such a manner that the signal PS is lower than the signal RS. It is easy to imagine how, by means of the signal DS it would have been possible to drive multiple transistors even of different kinds, perhaps by decoupling the control terminals by means of appropriate circuitry. In this case the term 'main conduction path' is understood in a broader sense and such as to give meaning to the `protection' concept widened to multiple transistors. With reference only to FIG. 4, in a method allowing very simple implementation the signals S1 and S2 are current signals and the signal PS is a voltage signal and the signal PS is obtained by means of connection in series of at least two junctions E-B (emitter-base) of a first T1 and a second T2 bipolar junction transistors to which are supplied respectively the signals S1 and S2 through two of their corresponding main conduction terminals E (emitters). Using transistors of different type and/or different topology it could be thought to supply the signals S1 and S2 e.g. to the collectors. Furthermore in the above mentioned case it is advantageous that the signal RS be a voltage signal and obtained by means of connection in series of at least two junctions E-B of two bipolar junction transistors T3 and T4 in such a manner as to have an analogous behaviour of the multiplier and the generator in case the four transistors T1, T2, T3, T4 are integrated together. As mentioned above, the present method lends itself to application also to other protection types such as for example that against the secondary breakdown for the bipolar junction transistors. Indeed, said complex limit corresponds, in accordance with a certain physical model, to the product of the collector current Ic or emitter Ie of the transistor and of the square of the voltage between the collector and the emitter Vce. The method suited to said type of protection calls furthermore for the phase to generate a third electrical signal essentially proportional to the voltage across the main conduction path (in this case the voltage Vce), and requires that in phase c) there are multiplied the first, second and third signals corresponding respectively in this case to the current Ic, the voltage Vce and the voltage Vce again. A block diagram of the protection circuit CPR in accordance with the present invention is shown in FIG. 1 inserted however in a voltage regulator. Said protection circuit for at least one power transistor PT having at least one control terminal G and two main conduction terminals D,S which identify a main conduction path D-S comprises at least: a) first detection means DM1 designed to generate a first electrical signal S1 essentially proportional to the current flowing in the path D-S of the transistor PT, b) second detection means corresponding in FIG. 1 to the set of a block DM2 and a detection transistor ST designed to generate a second electrical signal S2 essentially proportional to the voltage across the path D-S of the transistor PT, c) multiplier means MM receiving at input the first signal S1 and the second signal S2 and designed to generate an electrical product signal PS essentially corresponding at least to the product thereof, d) a generator RG of an electrical reference signal (RS), e) comparator means receiving at input the signal PS and the signal RS and designed to generate an electrical difference signal DS essentially corresponding to their difference, and f) control means designed to drive the control terminal G of the transistor PT on the basis of the signal DS so that the signal PS is lower than the signal RS. The comparator means and the control means are represented in FIG. 1 by means of the block CM on the assumption that the output stage of said block is capable of driving the transistor PT. In this case said output stage corresponds essentially to the control means. It is not excluded that in some cases the comparator means and the control means correspond to distinct circuit blocks as is evident for a common designer. Depending on the type of power transistor (BJT, MOS, ...) the manner of driving the control terminal (the base, the gate, ...) varies as is well known to those skilled in the art. It can be for example a current or a voltage driving. FIG. 2 shows the electrical diagram limited to the blocks MM, RG, CM of a first embodiment of the circuit in accordance with the present invention. It comprises a transconductance multiplier receiving at first inputs the signal S1, a voltage signal in this case, and at a second input the signal S2, in this case a current signal. The transconductance multiplier consists of two transistors T12 and T22 of the bipolar junction type connected in differential configuration. The first inputs correspond to the bases of these transistors while the second input is connected together with their emitters in such a manner as to determine their polarisation current. Their collectors are connected to an active load consisting of two transistors T11 and T21 of the bipolar junction type connected as a current mirror. The output of the multiplier (of current) is connected to the collector of the transistor T12 and is supplied to a first terminal of a load resistor RL having its other terminal connected to ground. Said resistor has the purpose of converting the output signal of a current signal to a voltage signal corresponding in this case to the signal PS. The comparator means and control means consist of a differential amplifier OA2 generating the signal DS making approximately the difference between the signals supplied to its inputs and operating therefore in linear zone. This receives at its inverting input the signal PS and at its non-inverting input the signal RS which is in this case a voltage signal generated by a reference voltage generator VR, e.g. a 'bandgap'. FIG. 3 shows in a voltage-current graph the ideal behaviour expected of the assembly of the means DM1, DM2 and MM by means of the curve MP. The curve MP is a hyperbolic arc corresponding to the dissipation limit of the transistor PT limited in current and voltage respectively to the values IM and VM corresponding to the current and voltage limits of the transistor. Normally the detector means DM1 and DM2 have a fairly linear behaviour which is thus close to ideal. The transconductance multiplier provides a multiplication only in a first approximation while in reality it provides, as is easy to calculate, a function of the hyperbolic type. This behaviour is shown in FIG. 3 by means of the curve CA. As may be noted, by using as dissipation limit the curve CA in certain operating conditions the capacities of the transistor are not completely utilised because the curve CA is below the curve MP while in other conditions the transistor can be taken to work outside the dissipation limits because the curve CA is above the curve MP. A first way to solve this additional problem is to choose a curve CA such as to be always below the curve MP accepting therefore wasting of a part of the dissipative capacity of the transistor. A second way consists of predistorting the signals S1 and/or S2 with a function corresponding to the inverse of the hyperbolic tangent. Naturally said predistorsion increases the complexity of the circuit. A third way consists of designing a multiplier having a transfer function more like a hyperbola and if possible having a circuitry not too complex. This way leads to the embodiment of the present invention having the partial electrical diagram shown in FIG. 4. In this third case, the signals S1 and S2 are current signals and the signal PS is a voltage signal, the means MM comprise at least two bipolar junction transistors T1,T2 having two corresponding junctions E-B connected in series, the signal PS corresponds essentially to the voltage across the two junctions E-B connected in series, and the current signals S1 and S2 are supplied to two emitter terminals E respectively of the two transistors T1,T2. The signal PS thus obtained corresponds essentially, as is easy to calculate, to the natural logarithm of the product of the signals S1 and S2. This fact does not affect the performance of the circuit because the signal PS is later compared with a constant reference signal and hence, to allow for the presence of the logarithm, it is sufficient to choose an appropriate value for said reference signal. In the circuit of FIG. 4 the signal RS is a voltage signal, the generator RG comprises at least two bipolar junction transistors T3,T4 having two corresponding junctions E-B connected in series, and the signal RS corresponds essentially to the voltage across the two junctions E-B connected in series. To the transistors T3,T4 is supplied the same polarisation current through the reference current generator RI. The transistors T3,T4 are connected as a diode and therefore could in principle be replaced by two actual diodes. The advantage of using two transistors is that in an integrated embodiment of the circuit the transistors T3,T4 can be provided symmetrically with the transistors T1,T2 and therefore optimise the behaviour of the multiplier on the basis of the dispersion and the variations in the electrical characteristics thereof. There are various ways of providing the blocks DM1 and DM2, i.e. generating electrical signals approximately proportional to the current in a branch and to the voltage between two nodes of a circuit. Deviation from ideal behaviour could consist e.g. of the presence of a constant addendum, imperfect behaviour of linearity, or intrinsic non-linearity of behaviour (non-linear function not deviating significantly from the linear function in the circuit operating range). Preferred embodiments usable e.g. with the circuit of FIG. 4 are discussed below with the aid of FIGS. 5 and 6. The circuit of FIG. 5 corresponds with the first detection means DM1 and comprises: a)a low resistance detection resistor R3 connected in series with the main conduction path D-S of the transistor ST, b)two symmetrical resistors R1,R2 having first terminals connected respectively to the terminals of the detection resistor R3, and c)a current mirror circuit MI1 having two inputs I1,I2 respectively connected to second terminals of the symmetrical resistors R1,R2. The signal S1 corresponds with the current extracted from one of the inputs of the circuit MI1 and specifically the input I1 and due to its unbalance. More precisely, between the input I1 and the output of the circuit of FIG. 4 is interposed the path D-S of an MOS transistor M1 acting as an output buffer. The resistor R3 must have a sufficiently low resistance so that the voltage drop thereon is small in comparison with the voltage VDS of the transistor PT over the entire circuit operating range. In an integrated embodiment of the present circuit a value of 15mΩ realised by means of a strip of "Metal" was chosen. The currents flowing in the circuit MI1 and the current corresponding to the signal S1 must also be small. If the mirror circuit MI1 is such as to cause flow to the inputs of equal currents the resistances of the resistors R1,R2 must be equal. The circuit MI1 consists of a pair of current generators I31,I41 supplying the collector current respectively to a pair of transistors T32,T42 connected as a current mirror and which in turn supply through the respective emitters the collector current respectively to another pair of transistors T31,T41 which are also connected as a current mirror. The emitters of the transistors T31,T41 are respectively connected to the inputs I1,I2 of the circuit MI1. The gate terminal of the transistor M1 is connected to the collector of the transistor T42. The circuit of FIG. 6 corresponds to the second detection means connected to the transistor PT and comprises a detection transistor ST of the same type as the transistor PT but having a smaller channel width-to-length ratio and having its control terminal G connected to the control terminal G of the transistor PT, its source terminal S connected to the corresponding terminal S of the transistor PT, and its drain terminal D to the corresponding terminal D of the transistor PT through at least one limitation resistor R4 (in FIG. 6 the series connection of two resistors R4 and R7) and comprises additionally third detection means DM3 designed to generate the signal S2 in such a manner that it is essentially proportional to the current flowing in the limitation resistor R4. The resistor R4 in combination with the dimensioning of the channel of the transistor ST serves to make the voltage VDS of the transistor ST much smaller than the voltage VDS of the transistor PT. In an integrated embodiment this ratio was chosen 20,000 times smaller than the value of the resistor R4 which is 150kΩ. With a net equation including the resistor R4 and the two transistors ST and PT, it is seen immediately that, under the adopted assumptions, there is an essentially proportional link between the voltage VDS of the transistor PT and the current flowing in the resistor R4. The third detection means DM3 are, in the circuit of FIG. 6, provided by means of the same circuitry solution used for the means DM1 and shown in FIG. 5. They include: a) a low resistance detection resistor R7 connected in series with the limitation resistor R4, b) two symmetrical resistors R5,R6 having first terminals respectively connected to the terminals of the resistor R4, and c) a current mirror circuit MI2 having two inputs I5,I6 connected respectively to second terminals of the resistors R5,R6. The signal S2 corresponds to the current extracted from one of the inputs of the circuit MI2, in particular the input I5, and due to its unbalance. More precisely between the input I5 and the output of the circuit of FIG. 5 is interposed the path D-S of an MOS transistor M2 acting as output buffer. The resistor R7 must have a very low resistance in comparison with the resistance of the resistor R4. In the integrated embodiment of the present circuit mentioned above a value of 1.2kΩ was chosen. The circuit MI2 consists of a pair of current generators I51,I61 supplying the collector current respectively to a pair of transistors T52,T62 connected as a current mirror and which in turn supply through the respective emitters the collector current respectively to another pair of transistors T51,T61 which are also connected as a current mirror. The emitters of the transistors T51,T61 are respectively connected to the inputs I5,I6 of the circuit MI2. The gate terminal of the transistor M2 is connected to the collector of the transistor T62. With a net equation including the resistors R5,R6,R7 it is immediately seen that, under the assumptions adopted above and the additional assumptions that R5 = R6 + R7 and that the mirror circuit MI2 is such as to cause equal currents to flow to the inputs, there is an essentially proportional link between the current in the resistor R4 and the current corresponding to the signal S2. Consequently there is an essentially proportional link between the voltage VDS of the transistor PT and the current corresponding to the signal S2. As mentioned above, the protection circuit in accordance with the present invention finds advantageous application in voltage regulators. FIG. 1 shows such a voltage regulator including at least one power transistor PT and one protection circuit CPR for at least said transistor. The latter exhibits an input VIN referred to the ground GND and an output VOUT. The drain terminal D of the transistor PT is connected to the input VIN through the means DM1 and the source terminal S of the transistor PT is connected directly to the output VOUT. To the input VIN is connected a first block B1 raising the voltage and supplying it to an output current generator OI having the purpose of driving the gate terminal G of the transistor PT with a voltage sufficiently high to hold it in conduction with changes in the output regulated voltage. Between the output VOUT and ground GND is connected a voltage divider consisting of the series connection of two resistive elements E1 and E2. The intermediate tap of the divider is connected to the inverting input of an operational amplifier OA1. The non-inverting input of the amplifier OA1 is connected to the output of a second block B2 generating a reference voltage for regulation, e.g. a bandgap. The output of the amplifier OA1 is connected to the terminal G of the transistor PT in such a manner as to regulate the output voltage in relation to the division ratio of the voltage divider E1,E2. Of course the protection circuit in accordance with the present invention can find application in many other integrated and unintegrated circuits.

Claims: Method for protection of at least one power transistor (PT) having at least one control terminal (G) and two main conduction terminals (D,S) identifying a main conduction path (D-S) and comprising the following steps: a) generation of a first electrical signal (S1) essentially proportional to the current flowing in said path (D-S), b) generation of a second electrical signal (S2) essentially proportional to the voltage across said path (D-S), characterised in that it further comprises the following steps: c) multiplication of at least said first (S1) and second (S2) signals finding an electrical product signal (PS) , d) comparison of said product signal (PS) with an electrical reference signal(RS) finding an electrical difference signal (DS), and e) driving of said control terminal (G) by means of said difference signal (DS) in such a manner that said product signal (PS) is smaller than said reference signal (RS). Method in accordance with claim 1 wherein said first (S1) and second (S2) signals are current signals and said product signal (PS) is a voltage signal and wherein said product signal (PS) is obtained by means of series connection of at least two junctions (E-B) of a first (T1) and second (T2) bipolar junction transistors to which are supplied respectively said first (S1) and second (S2) signals through two of their corresponding main conduction terminals (E). Method in accordance with claim 2 wherein said reference signal (RS) is a voltage signal and is obtained by means of series connection of at least two junctions (E-B) of two bipolar junction transistors (T3,T4). Method in accordance with claim 1 comprising further the step of generating a third electrical signal essentially proportional to the voltage across said path and wherein in step c) are multiplied at least said first, second and third signals. Protection circuit for at least one power transistor (PT) having at least one control terminal (G) and two main conduction terminals (D,S) identifying a main conduction path (D-S) and comprising at least: a) first detection means (DM1) designed to generate a first electrical signal (S1) essentially proportional to the current flowing in said path (D-S), b) second detection means (DM2,ST) designed to generate a second electrical signal (S2) essentially proportional to the voltage across said path (D-S), characterised in that it further comprises: c) multiplying means (MM) receiving at input said first (S1) and second (S2) signals and designed to generate an electrical product signal (PS) basically corresponding to the product at least of the latter, d) a generator (RG) of a electrical reference signal (RS), e) comparator means (CM) receiving at input said product signal (PS) and said reference signal (RS) and designed to generate an electrical difference signal (DS) basically corresponding to their difference, and f) control means (CM) designed to drive said control terminal (G) on the basis of said difference signal (DS) so that said product signal (PS) is lower than said reference signal (RS). Circuit in accordance with claim 5 wherein said first (S1) and second (S2) signals are current signals and said product signal (PS) is a voltage signal and in which said multiplying means (MM) comprise at least two bipolar junction transistors (T1,T2) having two corresponding junctions (E-B) connected in series and wherein said product signal (PS) corresponds basically to the voltage across the two junctions (E-B) connected in series and wherein said first (S1) and second (S2) signals are supplied to two main conduction terminals (E) respectively of said two transistors (T1,T2). Circuit in accordance with claim 6 wherein said reference signal (RS) is a voltage signal and in which said generator (RG) comprises at least two bipolar junction transistors (T3,T4) having two corresponding junctions (E-B) connected in series and wherein said reference signal (RS) corresponds basically to the voltage across the two junctions (E-B) connected in series. Circuit in accordance with claim 5 wherein said first detection means (DM1) comprise: a) a detection resistor (R3) with low resistance connected in series with said path, b) two symmetrical resistors (R1,R2) having first terminals respectively connected to the terminals of said detection resistor (R3), and c) a current mirror circuit (MI1) having two inputs (I1,I2) respectively connected to second terminals of said symmetrical resistors (R1,R2), and in which said first signal (S1) corresponds to a current extracted from one (I1) of the inputs of said current mirror circuit (MI1) and due to its unbalance. Circuit in accordance with claim 5 wherein said second detector means (DM2,ST) comprise a detection transistor (ST) of the same type as said power transistor (PT) but having a lower channel width-to-length ratio, having control terminal (G) connected to the control terminal (G) of said power transistor (PT), a first main conduction terminal (S) connected to the corresponding terminal (S) of said power transistor, and a second main conduction terminal (D) to the corresponding terminal (D) of the power transistor through at least one limitation resistor (R4) and comprising additionally third detection means (DM3) designed to generate said second signal (S2) in such a manner that it is essentially proportional to the current flowing in said limitation resistor (R4). Circuit in accordance with claim 9 wherein said third detector means (DM3) comprise: a) a low resistance detection resistor (R7) connected in series with said limitation resistor (R4), b) two symmetrical resistors (R5,R6) having first terminals respectively connected to the terminals of said limitation resistor (R4), and c) a current mirror circuit (MI2) having two inputs (I5,I6) respectively connected to second terminals of said symmetrical resistors (R5,R6), and wherein said second signal (S2) corresponds to a current extracted from one (I5) of the inputs of said current mirror circuit (MI2) and due to its unbalance. Voltage regulator comprising at least one power transistor (PT) and one protection circuit for said at least one transistor in accordance with one of claims 5 to 10.

Inventors: BONTEMPO GREGORIO, COLLETTI PAOLO, GARIBOLDI ROBERTO, PULVIRENTI FRANCESCO, BONTEMPO, GREGORIO, COLLETTI, PAOLO, GARIBOLDI, ROBERTO, PULVIRENTI, FRANCESCO
Applicants: CONS RIC MICROELETTRONICA, ST MICROELECTRONICS SRL, CO.RI.M.ME. CONSORZIO PER LA RICERCA SULLA MICROELETTRONICA NEL MEZZOGIORNO, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: G05F1
IPC codes: H01L21, H01L29, H01L27, H03K17, H02H7, G05F1